The present invention relates to wireless power transfer.
The use of wireless power supply systems continues to grow. The most common wireless power supply systems use electromagnetic fields to wirelessly transfer power from a wireless power supply to wireless power receiver associated with a remote device, such as a cell phone, a smart phone, a media player or other electronic device. There are a number of different types of wireless power supply systems. For example, many conventional systems use a primary coil in the wireless power supply and secondary coil in the wireless power receiver of the remote device. The primary coil generates an electromagnetic field that emanates from the wireless power supply. The wireless power receiver includes a secondary coil that can be placed within the electromagnetic field generated by the primary coil. When the remote device is placed within sufficient proximity to the wireless power supply, the electromagnetic field induces power within the secondary coil that can be used by the remote device, for example, to power and/or charge the remote device. These types of systems typically provide optimal performance when the primary coil and the secondary coil are relatively close to one another. For this reason, these types of systems are often referred to as “close-coupled” systems.
A number of conventional wireless power supply systems have been configured to efficiently provide power when the primary coil and the secondary coil are farther apart than normally acceptable for efficient use of close-coupled systems. Given that they can efficiently transfer power at distances greater than close-coupled systems, these types of wireless power transfer systems are often referred to as “mid-range” systems. A typical mid-range wireless power transfer system relies on technology disclosed over 100 years ago by Nicola Tesla (see for example, U.S. Pat. No. 685,012, which issued on Oct. 22, 1901).
With a typical mid-range system, the power transfer system includes a pair of resonators that are arranged between or otherwise near the primary coil and the secondary coil. Each resonator is configured to include an inductor and a capacitor, and does not include any additional significant load. This keeps the impedance at the resonant frequency to a minimum which maximizes the resonating current between the capacitor and inductor. The current in the inductor, in turn, amplifies the wireless power signal induced within the resonator. Given their ability to amplify signals, the resonators can function as a bridge for extending the range of the wireless power supply system. In use, the primary coil generates an electromagnetic field that induces power in the first resonator, the first resonator generates an amplified electromagnetic field that induces power in the second resonator and the second resonator generates an amplified electromagnetic field that induces power in the secondary coil. For example, FIG. 1 illustrates one embodiment of a known wireless power supply system. The wireless power system of FIG. 1 includes a wireless power supply and a wireless receiver. The wireless power supply includes a connection to a mains input, an AC/DC power supply, an inverter for switching the DC power to AC, a tank circuit including a capacitor and an inductor L1. When energized the tank circuit inductor L1 couples to the isolated resonator circuit including the inductor L2 and the capacitor. The wireless receiver includes an isolated resonator circuit with an inductor L3 and a capacitor that couples with the isolates resonator circuit of the wireless power supply. The isolated resonator circuit of the wireless receiver relays power to the secondary circuit of the wireless receiver. The secondary circuit of the wireless receiver includes a secondary inductor L4, a capacitor, a rectifier, a controller and a load.
Although the use of resonators typically provides improved efficiency in mid-range environments, resonators can reduce efficiency when the wireless power supply and the remote device are too close. It is also possible for a resonator to relay more available power, leading to higher voltages than desired in some applications. This can lead to reduce the system's overall efficiency, generate significant heating and produce excessive voltages and circulating currents at the receiver.